The present invention relates generally to a finder optical system comprising an image-inversion optical subsystem, and more particularly to a finder optical system which is used on cameras, video cameras, etc., so that the inverted image of an object formed through an objective optical subsystem can be viewed in the form of an erect image using the image-inversion optical subsystem.
Among finder optical systems used on cameras, etc., there is well known a so-called real-image type finder wherein a primary real image formed through an objective optical subsystem is converted through an image-inversion optical subsystem to an erect image, which is then viewed through an ocular optical subsystem. The real-image type finder has the merit of turning back an optical path simultaneously with the erection of the image through the image-inversion optical subsystem, thereby reducing the size of the whole of the optical system. For this reason, this optical system is used on most of handy, easy-to-carry and slimmed down cameras now in demand. More recently, the finder optical system is ever-more reduced in size by incorporating a roof surface in the image-inversion optical subsystem, thereby enabling an erect image to be formed in an ever-smaller space.
With recent demands for camera or video size reductions, however, further size reductions of the finder optical system are required especially in terms of its size in the entrance direction or its size in the so-called thickness direction of a camera. The real-image type finder may be reduced in size by turning back its optical path. For the image-inversion optical subsystem, however, some constraints are imposed on the location of reflecting surfaces thereof, because unless the image-inversion optical subsystem is located between the objective optical subsystem where light beams converge with decreasing ray height and the ocular optical subsystem, it is then difficult to acquire the required number of reflecting surfaces. In other words, refracting lenses for the objective, and ocular optical subsystems have to be in the entrance direction of the first reflecting surface where the entrance optical axis is first bent and in the exit direction of the final reflecting surface where the optical path is again made parallel with the entrance optical axis, and so the size of such refracting lenses comes to the size of the image-inversion optical subsystem in the thickness of the direction of the camera. Thus, some restraints are also imposed on making the real-image type finder thin.
On the other hand, a prior art image-inversion optical subsystem has reflecting surfaces constructed of plane surfaces, and so has generally no power. For this reason, some proposals have been made to impart power to the reflecting surfaces of a prism or mirror acting as the image-inversion optical subsystem, thereby allowing it to function an objective, and ocular optical subsystem for the purpose of achieving compactness. In such an optical system, the image-inversion optical subsystem functions partly as the objective, and ocular optical subsystem. It is thus to be understood that the image-inversion optical subsystem used herein is included in the objective, and ocular optical subsystem, and the means having the image-inversion action is referred to as the "image-inversion means" and the member to form the "image-inversion means" is especially called a "reflection optical subsystem with powers imparted to reflecting surfaces".
JP-A 8-248481 discloses a real-image type finder wherein rotationally asymmetric curved surfaces are used for the reflecting surfaces of a prism. This publication teaches that aspheric or toric surfaces may be used as the curved surfaces. However, only rotationally symmetric aspheric surfaces are described as the curved surfaces. A toric surface is generally a surface with respect to two coordinate axes or, in another parlance, is not an asymmetric curved surface.
JP-A 9-152646 discloses a real-image type finder wherein rotationally asymmetric curved surfaces are used for the reflecting surfaces of a prism. In Example 1, the objective optical subsystem is made up of only one prism having a positive power. The half angle of view shown there is 13.0.degree. in the Y-direction and 8.8.degree. in the X-direction. Example 2 shows an objective optical subsystem consisting of one negative refracting lens, one positive refracting lens and one prism having a positive power.
JP-A 10-68887 discloses a binocular wherein rotationally asymmetric curved surfaces are used for the reflecting surfaces of a prism. Example 1 is directed to a single image-formation system wherein the objective optical subsystem is made up of two prisms. The half angle of view shown there is 6.55.degree. in the horizontal direction and 8.73.degree. in the vertical direction. No detailed makeup of the ocular optical subsystem is described.
JP-A 10-197705 discloses a binocular with rotationally asymmetric curved surfaces used for the reflecting surfaces of a prism. In Examples 1 to 3, the objective optical subsystem is made up of two refracting lenses and one prism while the ocular optical subsystem is made up of one prism. The half angle of view shown there is 4.36.degree. to 6.10.degree. in the horizontal direction and 3.27.degree. to 4.70.degree. in the vertical direction. In Example 4, the objective optical subsystem and the ocular optical subsystem are each made up of one prism. The half angle of view is then 6.7.degree. in the horizontal direction and 5.0.degree. in the vertical direction.
JP-A 10-197796 discloses a real-image type finder optical system with a rotationally asymmetric curved surface used in an image-inversion optical subsystem. Any design example is not shown in most examples, and so its performance, size, etc. remain unclear.
However, such prior arts have various problems as explained below.
In the system set forth in JP-A 8-248481, powers are imparted to the reflecting surfaces of the prism. Since the reflecting surfaces are decentered or inclined with respect to axial chief rays, there are rotationally asymmetric decentration aberrations. However, these aberrations cannot be corrected by rotationally symmetric aspheric surfaces. Even at toric surfaces, sufficient correction cannot be made for skew rays. This publication fails to disclose any means for solving these correction problems whatsoever. It is thus believed that any high-performance is not achieved.
In the system set forth in JP-A 9-152646, such decentration aberrations as mentioned above are reduced by using rotationally asymmetric curved surfaces for the reflecting surfaces of a prism. In Example 1, nonetheless, the decentration aberrations remain undercorrected even by use of such rotationally asymmetric curved surfaces because of a large angle of reflection at the first reflecting surface located in the objective optical subsystem; no sufficient performance is again obtained. In addition, this finder optical system has a very narrow angle of view, and so have very limited applications. In Example 2, as many as two refracting lenses are unavoidably used in the objective optical subsystem and, hence, any sufficient size reduction is not achieved, because of a failure in taking full advantage of the powers of the reflecting surfaces in spite of being a single-focus finder.
In the system set forth in JP-A 10-68887, five rotationally asymmetric reflecting surfaces are used to make correction for decentration aberrations. However, the examples are all directed to a binocular with a narrow angle of view. Most cameras, whether they are of the single-focus type or of the zoom type, include a wide-angle system (with a focal length of about 25 mm to about 40 mm and a half angle of view of 28.4.degree. to 40.8.degree. as calculated on the basis of 35-mm film), and so this system cannot immediately be applied thereto. An increased angle of view causes increases in the effective areas of the reflecting surfaces of a prism, which in turn cause an increase in the size of the prism with increasing amounts of aberrations due to an increased angle of reflection at the reflecting surfaces. In the scope of the invention disclosed in this publication, it is thus difficult to achieve a finder having a large angle of view. In addition, this publication fails to disclose the makeup of the ocular optical subsystem or take the whole size of the optical system into consideration.
In the system set forth in JP-A 10-197705, too, rotationally asymmetric reflecting surfaces are used. However, this system is an optical system for binocular purposes as in the above prior arts, and so can hardly be applied to a finder due to its narrow angle of view.
JP-A 10-197796 teaches how the prism should be located and constructed so as to enable the finder system to be reduced in size. However, this is little achievable because performance is not taken into account.
Thus, all the prior arts have problems in connection with performance and size, and never until now is any compact yet high-performance finder capable of solving these problems at the same time achievable.